The family of inhibin-related proteins currently consists of at least four groups of members: inhibins, activins, and two splice variants of follistatin-1 (315 and 288 amino acids). Inhibins and activins are members of the transforming growth factor (TGF)-beta superfamily and function with opposing actions in a variety of capacities in paracrine and autocrine regulation of both reproductive and nonreproductive organs including the liver, kidney, adrenal glands, bone marrow, placenta, anterior pituitary, and brain (Ying, S. Y., et al., Proc. Soc. Exp. Biol. Med. 214:114–122 (1997); Mather, J. P., et al., Proc. Soc. Exp. Biol. Med. 215:209–222 (1997)). Although the follistatins are not closely related to the TGF-beta family, they still play a major role in the follical stimulating hormone (FSH) synthetic pathway by increasing estradiol production and by functioning directly as high affinity activin-binding proteins. Inhibins, activins, and follistatin-I were all initially identified as regulators of pituitary FSH secretion, but have more recently been further characterized to function as growth factors, embryo modulators, and immune factors (Petraglia, F. Placenta 18:3–8 (1997)). In addition, each of these factors is involved with the regulation of gonadotropin biosynthesis and secretion, ovarian and placental steroidogenesis, and oocyte and spermatogonial maturation (Halvorson, L. M. and DeCherney, A. H. Fertil. Steril. 65:459–469 (1996)).
FSH is a vital component of the regulatory cascade governing development of human oocytes. Primary oocytes in newborns are arrested in the prophase stage of Meiosis I and are surrounded by a 1–2 cell thick layer of follicle cells constituting a structure termed the primordial follicle. In concert with other factors, stimulation of the primordial follicle with FSH initiates its progression to the more complex structures designated the developing and antral follicles (Ueno, N., et al., Proc. Natl. Acad. Sci. USA 84:8282–8286 (1987); Robertson, D. M., et al., Biochem. Biophys. Res. Comm. 149:744–749 (1987)). The antral follicle consists of an enlarged oocyte surrounded by an increased number of follicle cells, a zona pellucida, cortical granules, and a fluid-filled cavity termed the antrum. It is in this state that thousands of developing oocytes are maintained until puberty. Each month following this point, a surge in the local concentration of several additional hormones and other factors, primarily leuteinizing hormone (LH), stimulates accelerates the growth of roughly 15–20 of the developing follicles in the ovary. Only one of these structures will ultimately complete the developmental progression of its enclosed oocyte to the metaphase stage of Meiosis II. The single stimulated follicle will then continue to enlarge until it bursts at the surface of the ovary and releases the oocyte, still surrounded with a coating of follicle cells, for potential fertilization (Bornslaeger, E. A., et al., Dev. Biol. 114:453–462 (1986); Masui, Y. and Clarke, H. J. Int. Rev. Cytol. 57:185–282 (1979); Richards, J. S. Recent Prog. Horm. Res. 35:343–373 (1979)).
Follistatin also plays a central role in the above-described process of follicle development. Follistatin binds stoichiometrically to activins and, as a result, inhibits the activin-induced augmentation of FSH-release from cultured pituitary cells (Kogawa, K., et al., Endocrinology 128:1434–1440 (1991)). Further evidencing a feedback mechanism, cultured granulosa cells produce and secrete follistatin in response to treatment with FSH (Saito, S., et al., Biochem. Biophys. Res. Comm. 176:413–422 (1991); Klein, R., et al, Endocrinology 128:1048–1056 (1991)). Furthermore, it has been determined by synthesizing the results of a number of studies, that follistatin, activin, FSH, LH, and other factors function in concert in a variety of interrelated mechanisms to regulate many developmental processes, including the development of follicles. For example, in the presence of FSH, activin can augment both LH receptor expression and progesterone production by rat granulosa cells (Sugino, H., et al., Biochem. Biophys. Res. Comm. 153:281–288 (1988)). In addition, activin can significantly enhance the ability of granulosa cells to express FSH receptor and produce inhibin even in the absence of FSH (Nakamura, T., et al., Biochim. Biophys. Acta 1135:103–109 (1992); Sugino, H., et al, supra; Hasegawa, Y., et al., Biochem. Biophys. Res. Comm. 156:668–674 (1988)). These and other studies provide support for the idea that follistatin and activin play important roles in the regulation of granulosa cellular differentiation.
In addition to the many well-characterized effects which follistatin, activin, and inhibin elicit on the regulation of various developmental processes in the reproductive system, a large number of studies have more recently begun to define regulatory roles for these molecules in a variety of other tissues and systems. For example, during early embryonic development in Xenopus laevis, the action of activin A in developing targets of ciliary ganglion neurons is regulated by localized expression of follistatin (Hemmati-Brivanlou, A. and Melton, D. A. Nature 359:609–614(1992); Hemmati-Brivanlou, A. and Melton, D. A. Cell 77:273–281(1994)). In addition, overexpression of follistatin leads to induction of neural tissue (Hemmati-Brivanlou, A., et al., Cell 77:283–295 (1994)). In the mouse, follistatin mRNA is first detected on embryonic day 5.5 in the deciduum, and, subsequently, in the developing hindbrain, somites, vibrissae, teeth, epidermis, and muscle (van den Eihnden-van Raaij, A. J. M., et al, Dev. Biol. 154:356–365 (1992); Albano, R. M., et al., Development 120:803–813 (1994); Feijen, A., et al., Development 120:3621–3637 (1994)). Evidence of the relative importance of such a varied expression of follistatin is provided by Matzuk and colleagues (Nature 374:360–363 (1995)) who demonstrate that follistatin-deficient mice are retarded in their growth, have decreased mass of the diaphragm and intercostal muscles, shiny taut skin, skeletal defects of the hard palate and the thirteenth pair of fibs, their whisker and tooth development is abnormal, they fail to breathe, and die within hours of birth. Since the defects in mice deficient in follistatin are far more widespread than in mice deficient in activin, Matzuk and coworkers (supra) suggest that follistatin may modulate the cell growth and differentiation regulatory actions of additional members of the TGF-beta superfamily.
Thus, there is a need for polypeptides that function as regulators of reproductive development, embryonic development, and cell growth and differentiation since disturbances of such regulation may be involved in disorders relating to reproduction and the regulation of cell growth and differentiation. Therefore, there is a need for identification and characterization of such human polypeptides which can play a role in detecting, preventing, ameliorating or correcting such disorders.